Using the expression extra cash, left over cash, or discretionary income would have been more better than using excess money in your expression which sounds more ambiguous to me, it sounds more like one needs more than enough prior to starting bitcon investment since anyone can come in with as little as what can be called left over money when our basic needs has been taken care of.
For me there is no cash that is left over cash because human needs are unsatisfiable. The cash you call left over is only that you have not really though of investing it or use it for a particular need. What people call leftover cash can quickly be inverted to Bitcoin andnsabe for the future because there is a saying that 'the money you save today will save you tomorrow' this is why I have chosen to save my own money in Bitcoin. Investing in bitcoin is something you need to have a set goal. First you need to have a source of income and set outside the cash for your basic needs and also set outside emergency fund so that you will have a relaxed mind as you will be able to keep your investment growing. As I was told in this forum that using DCA is the best method of growing your bitcoin investment which have been working for me and also to start quickly as possible as procrastination may lead to had I know.
You are not saving your money in bitcoin rather you are investing it and the money you use in investing into Bitcoin is your discretionary income which is also your leftover fund, if you are not investing with your leftover fund you will definitely sell your bitcoin in a short while because the money which you use to invest into Bitcoin is the money you will be needing soon that's why we are advised to use our leftover fund to invest in bitcoin that's the money we won't be using in the next 4-10 or more.